Devices of the type described herein serve generally to couple mechanically percutaneous prosthetic members for attachment of artificial eyes, ears, limbs, and, in particular, teeth to bone. Typically, such devices comprise two connectable main components; an implant component for integration with bone which serves an anchoring function and a separate support component that traverses epithelial tissue to which the prosthesis may be attached and subsequently coupled to the implant.
The tenacity with which the bone is able to retain the implant in situ is one factor which determines the success of devices of this type. This factor depends on the stability of the implant relative to the bone during the early healing phase following implant insertion and on the transfer of a small amount of stress from the implant to the bone in order to maintain bone and to increase bone density and strength.
Prior art proposals focus largely therefore on the means by which the implant is anchored within the bone.
One such proposal relating specifically to dental prostheses is disclosed in Int. J. Oral Surg. 1981; 10; 387-416, Adell et at. In that arrangement, screw-threaded cylindrical implants are provided upon which a dental bridge may be connected through a plurality of connectable supporting elements. Cooperation between the implant and the bone is provided by a bore in the jaw which is tapped to provide screw-threading complementary to the implant. A similar arrangement is proposed by Niznick in Implantology, November, 1983 Volume 73, Number 11, pages 13-15 in which the threaded cylindrical implant is provided near its base with channels through which bone is permitted to grow in order to enhance co-operation between the bone and the implant.
Another means by which an implant may be anchored in position is taught by Pilliar in U.S. Pat. No. 3,855,638. This reference describes a surgical prosthetic device with a porous metal coating. The dental implant taught by Pilliar comprises a cylindrical red having a porous coating on at least the areas of the implant in contact with and adjacent to bone. Such a porous coating allows for tissue ingrowth and tissue ossification. This reference suggests that the implant may also have a porous coating on the entire surface of the implant to encourage soft tissue growth in regions contacting such soft tissue. The provision of a porous coating on the entire implant surface provides, it is stated, a very rigid structure.
Further, in U.S. Pat. No. 4,223,412, Aoyagi et al. teach an implant having dental application comprising a pair of arms which extend into the receiving implant cavity. The entire surface of the implant is coated with a mixture of ceramics and hydroxyapatite, a mixture which has been shown to have excellent compatibility with living tissues. Such a coating has affinity to living tissue which allows for fixation of the implant.
Tapered implant devices are known. Greenfield (U.S. Pat. No. 943,113) provides a tapered implant frame for an artificial tooth. This design comprises a ribbed frame to allow for bone growth around and throughout the frame in order to secure the implant in position. Kiernan et al. (U.S. Pat. No. 2,857,670) also disclose a tapered implant for dental application. The way in which this implant is anchored in position, however, is by means of a plurality of serrations to allow bone growth throughout the implant. Outwardly extending studs are also provided on the implant for the purpose of gripping the surrounding bone structure.
Spector et al., in U.S. Pat. No. 4,164,794, disclose prosthetic devices having dental application. Specifically, there is disclosed a device having a tapered body, the entire surface of which is coated with a porous coating. However, it has recently been shown by Deporter et al. (J. Dent. Res. 67, 1190-1195, 1988) that the risk of infection from dental plaque may be increased when the entire length of the root component of such an implant is porous-coated. This risk is particularly increased when regions of the porous coating become directly exposed to the oral cavity.
It would be desirable to have an implant which, in use, possesses optimum fixation between the implant and the bone together with a reduced probability of the occurrence of infection.